1. Field of the Invention
The present invention relates to a hand-held instrument for measuring reflections on a colored control field of a printed sheet.
2. Description of the Prior Art
Instruments of this type are used in the printing industry in the form of color measuring instruments and top color illumination densitometers for controlling the color flow in printing machines, particularly in multi-color screen printing. Usually, the measurements are not taken within the printed image but on so-called printing control strips which are printed along the margins of the printed sheets. These printing control strips are normally arranged along the entire width of the printed sheet so as to permit the control of the color flow for each printed location. To enable the requirement of the colored printing inks in the various zones of the printing machine to be determined, measurements must be made for metering the color flow or measuring the color density, for which purpose various instruments have been used. The printing control strips are sub-divided into a succession of small control fields for the individual color zones. The control fields may be printed integrally over the entire surface or in dots, or two or more colors may be printed over each other for controlling their co-action. It is desirable to keep the control fields as small as possible because, in the first place, the marginal control strips reduce the size of the useful printed sheet and, secondly, it is desired to place several control fields within the color zone width of 30 to 40 mm to obtain the necessary measurement values for each of the four to five printing ink colors used. Most printing control strips have control fields of a size not exceeding 6.times.6 mm, and the size of the usually round measuring apertures of densitometers is between 3 mm and 3.5 mm. The limited size of the control fields poses a special problem for commercially available hand-held measuring instruments because the aperture of the measuring head must be focussed as accurately as possible over the center of the control field and the measuring heads are substantially larger than the control fields. This makes it impossible visually to control the positioning of the measuring aperture when the measuring head is placed on the control field.
The size of the measuring head is determined by various factors. First, a number of structural parts must be built into the measuring head, such as a source of the measuring light, a receiver of the measuring light, including usually several photo elements with filters, and light guiding channels, including optical components, for guiding the light from the source and the reflected light from the control field to the receiver. Various standards furthermore require particular arrangements for the structural parts. For example, top color illumination densitometers may be standardized for a 45.degree./0.degree. measuring geometry or a 0.degree./45.degree. measuring geometry, which means that the illuminating measuring light is selectively guided and the reflected light is collected at 45.degree. and 0.degree. relative to the measuring plane. This measuring geometry in conjunction with the similarly standardized small aperture angles of +/-5.degree. for the illuminating and reflected light means that the size of the measuring head is about 30 mm in diameter and 30 mm to 50 mm in height, and the measuring aperture is normally located on or near the center axis of the measuring head. This applies to measuring heads of round, quadratic or rectangular cross section. To make it possible to center such measuring heads with their measuring apertures accurately over the center of the control fields, known densitometers of this type have finders. The finder is affixed to the bottom plate of the instrument and is comprised of a thin plate defining a usually round hole of the same size as the measuring aperture of the measuring head. The measuring instrument proper is hinged or otherwise connected to the bottom plate so that it may be lowered. Thus, the finder may be visibly controllably positioned over the center of the control field to be measured, while the instrument is raised. The measuring head may then be lowered, which in more recent instruments has not been done manually but with a motor-driven measuring head.
These known instruments have substantially three disadvantages. They are technically complex and expensive in the first place and, secondly, they are not user friendly because they require a very careful positioning of the finder and the measuring instrument. The most serious disadvantage is the fact that the user cannot view the selected control field during the measurement because it is covered by the measuring head. It is, therefore, impossible to notice displacements of the measuring instrument relative to the control field, which may readily occur during the lowering of the instrument. This may result in false measuring results when the control field is measured not at its center but at a margin or if it is even partially or completely out of range.
Another type of hand-held measuring instruments has a rigidly mounted measuring head. The accurate positioning of such an instrument with respect to a spot to be measured is achieved by strongly tapering the measuring head downwardly. U.S. Pat. No. 4,264,210 discloses a dot measuring instrument with an arm carrying a light receiver unit in the form of a vertical cylinder. Accurate positioning of this instrument can be obtained only if the measuring plane is viewed under an oblique angle. If the user looks down more or less vertically on the measuring instrument and the measuring plane, accurate positioning is impossible because the carrying arm and the upwardly widening shape blocks the view from above. It must be noted in this respect that all such known instruments have light receiving units which widen towards the top, particularly instruments which are designed not only to test films with transmitted light but also copies with top illumination. The instruments must have two light guiding channels for measuring copies with top illuminations, one channel portion for guiding the measuring light illuminating the spot to be measured and another channel portion for guiding the reflected light.
Instruments with such rigid measuring heads cannot be used for measuring printed sheets, and particularly the control fields of printing control strips, because it is preferred to look down vertically when visually judging the printing quality. This more or less vertical orientation of viewing the printed sheet has the purpose to exclude the disturbing influence of surface gloss and reflections from the visual judgment of the colors. For this purpose, testing tables for printed sheets or printing control strips have obliquely positioned table tops on which the printed sheets are placed. This enables a person standing in front of the table to look more or less perpendicularly down on the table. Therefore, the densitometers used for this purpose comprise the above-described combination of movable measuring head and finder. The instruments with a fixed measuring head are used almost exclusively for simple black-and-white densitometers. The measurements are made at selected spots on the film or the black-and-white picture so that the difficulty of centering the measuring aperture geometrically accurately over a limited control field under the aggravated conditions of color measurement do not exist.